New developments in SAR test methods are bringing stricter limits and requirements, but more-accurate results.

Concern about human exposure to radio frequencies (RF) is not new. Ensuring the safety of RF devices is the primary motivation for new standards and test methods. The concept of specific absorption rate (SAR) has been around for many years, but recent developments have improved test methods. This article provides an overview of the current limits and test methods for SAR. Standards, specifications, and requirements are also discussed.

Health Effects
The heating effect from RF devices causes the most concern from an RF safety point of view. The human body counters local heating by thermoregulation (blood flow through the affected organs). The eyes and male testes are particularly susceptible to RF heating because these organs have no direct blood supply and, hence, no way of dissipating heat. The heating effects in biological tissue escalate with the increase in frequency, although the heat’s penetration depth decreases.
With the proliferation of cellular phones, most RF safety concerns have focused on RF absorption by the head, particularly from mobile handsets. The dose of RF exposure is linked to exposure time: maximum SAR is normally averaged over a 6-minute period during the 24-hour day.

Some concerns have focused on other effects of RF exposure. Most communications systems are pulse-like in nature, and their effects on brain function have been discussed recently. For example, the global system for mobile communications (GSM) frame rate, at 8.33 Hz, is close to that characteristic of alpha waves in the brain. Although there is no conclusive proof of such effects, considerable research is currently examining the effects of RF. Much of the research in this area was sparked by a report published by the Independent Expert Group on Mobile Phones, chaired by Sir William Stewart. The report, released in April 2000, is also known as the Stewart Report.

In the UK, nearly £7.4 million ($11.7 million) has been allocated from both government and industry sources to research the effects of RF. The LINK Mobile Telecommunications and Health Research (MTHR) Programme will be funded over a three-year period. The Programme Management Committee (PMC) was set up to advise on this research program. To date, PMC has published two calls for research proposals, and the first group of the projects is now under way. PMC has decided to issue a third call for research proposals. Much of this program’s research addresses the biological effects of RF on the human body. Currently, widely reproducible studies of RF effects on biological cells are not available.

The SAR Index
SAR is an index that quantifies the rate of energy absorption in biological tissue. SAR is expressed in watts per kilogram (W/kg¬1) of biological tissue. SAR is generally quoted as a figure averaged over a volume corresponding to either 1 g or 10 g of body tissue. The SAR of a wireless product can be measured in two ways. It can be measured directly using body phantoms, robot arms, and associated test equipment, or it can be mathematically modeled. Mathematical modeling of a product for SAR can be costly, and it can take as long as several months. Using conventional SAR test methods, a dual-band GSM 900 and GSM 1800 handset takes about one day to test to current standards.

SAR Limits
Several organizations have set exposure limits for acceptable RF safety via SAR levels. The International Commission on Non-Ionizing Radiation Protection (ICNIRP) was launched as an independent commission in May 1992. This group publishes guidelines and recommendations related to human RF exposure.

For the American National Standards Institute (ANSI), the RF safety sections now operate as part of the Institute of Electrical and Electronic Engineers (IEEE). IEEE recently wrote one of the most important publications for SAR test methods.1
In the UK, the National Radiological Protection Board (NRPB) sets SAR limits. SAR limits are expressed for two different classes of people: workers (occupational/controlled exposure) and the general population (uncontrolled exposure). Because the general-population exposure is considered to be uncontrolled, the limit for this group is five times more stringent than the limit for the workers, whose environment and exposure can be monitored and controlled.

The limits are defined for exposure of the whole body, partial body (e.g., head and trunk), and hands, feet, wrists, and ankles. SAR limits are based on whole-body exposure levels of 0.4 W/kg¬1 for workers and 0.08 W/kg¬1 for the general population. Limits are less stringent for exposure to hands, wrists, feet, and ankles. There are also considerable problems with the practicalities of measuring SAR in such body areas, because they are not normally modeled. In practice, measurements are made against a flat phantom, providing a conservative result.

Most SAR testing concerns exposure to the head. For Europe, the current limit is 2 W/kg¬1 for 10-g volume-averaged SAR. For the United States and a number of other countries, the limit is 1.6 W/kg¬1 for 1-g volume-averaged SAR. The lower U.S. limit is more stringent because it is volume-averaged over a smaller amount of tissue. Australia, Canada, and New Zealand have adopted the more-stringent U.S. limits of 1.6 W/kg¬1 for 1-g volume-averaged SAR. Japan and Korea have adopted 2 W/kg¬1 for 10-g volume-averaged SAR, as used in Europe.

Kelly Classic, Certified Medical Physicist
Electromagnetic radiation consists of waves of electric and magnetic energy moving together (that is, radiating) through space at the speed of light. Taken together, all forms of electromagnetic energy are referred to as the electromagnetic spectrum. Radio waves and microwaves emitted by transmitting antennas are one form of electromagnetic energy. Often the term electromagnetic field or radiofrequency (RF) field may be used to indicate the presence of electromagnetic or RF energy.

An RF field has both an electric and a magnetic component (electric field and magnetic field), and it is often convenient to express the intensity of the RF environment at a given location in terms of units specific for each component. For example, the unit “volts per meter” (V/m) is used to measure the strength of the electric field and the unit “amperes per meter” (A/m) is used to express the strength of the magnetic field.

RF waves can be characterized by a wavelength and a frequency. The wavelength is the distance covered by one complete cycle of the electromagnetic wave, while the frequency is the number of electromagnetic waves passing a given point in one second. The frequency of an RF signal is usually expressed in terms of a unit called the hertz (Hz). One Hz equals one cycle per second. One megahertz (MHz) equals one million cycles per second. Different forms of electromagnetic energy are categorized by their wavelengths and frequencies. The RF part of the electromagnetic spectrum is generally defined as that part of the spectrum where electromagnetic waves have frequencies in the range of about 3 kilohertz (3 kHz) to 300 gigahertz (300 GHz).

Probably the most important use for RF energy is in providing telecommunications services. Radio and television broadcasting, cellular telephones, radio communications for police and fire departments, amateur radio, microwave point-to-point links, and satellite communications are just a few of the many telecommunications applications. Microwave ovens are a good example of a noncommunication use of RF energy. Other important noncommunication uses of RF energy are radar and for industrial heating and sealing. Radar is a valuable tool used in many applications from traffic enforcement to air traffic control and military applications. Industrial heaters and sealers generate RF radiation that rapidly heats the material being processed in the same way that a microwave oven cooks food. These devices have many uses in industry, including molding plastic materials, gluing wood products, sealing items such as shoes and pocketbooks, and processing food products.

The quantity used to measure how much RF energy is actually absorbed in a body is called the specific absorption rate (SAR). It is usually expressed in units of watts per kilogram (W/kg) or milliwatts per gram (mW/g). In the case of whole-body exposure, a standing human adult can absorb RF energy at a maximum rate when the frequency of the RF radiation is in the range of about 80 and 100 MHz, meaning that the whole-body SAR is at a maximum under these conditions (resonance). Because of this resonance phenomenon, RF safety standards are generally most restrictive for these frequencies.
Biological effects that result from heating of tissue by RF energy are often referred to as “thermal” effects. It has been known for many years that exposure to very high levels of RF radiation can be harmful due to the ability of RF energy to rapidly heat biological tissue. This is the principle by which microwave ovens cook food.

Tissue damage in humans could occur during exposure to high RF levels because of the body’s inability to cope with or dissipate the excessive heat that could be generated. Two areas of the body, the eyes and the testes, are particularly vulnerable to RF heating because of the relative lack of available blood flow to dissipate the excessive heat load. At relatively low levels of exposure to RF radiation, that is, levels lower than those that would produce significant heating, the evidence for harmful biological effects is ambiguous and unproven. Such effects have sometimes been referred to as “nonthermal” effects. It is generally agreed that further research is needed to determine the effects and their possible relevance, if any, to human health.

In general, however, studies have shown that environmental levels of RF energy routinely encountered by the general public are typically far below levels necessary to produce significant heating and increased body temperature. However, there may be situations, particularly workplace environments near high-powered RF sources, where recommended limits for safe exposure of human beings to RF energy could be exceeded. In such cases, restrictive measures or actions may be necessary to ensure the safe use of RF energy.

Some studies have also examined the possibility of a link between RF and microwave exposure and cancer. Results to date have been inconclusive. While some experimental data have suggested a possible link between exposure and tumor formation in animals exposed under certain specific conditions, the results have not been independently replicated. In fact, other studies have failed to find evidence for a causal link to cancer or any related condition. Further research is underway in several laboratories to help resolve this question.

In 1996, the World Health Organization (WHO) established a program called the International EMF Project that is designed to review the scientific literature concerning biological effects of electromagnetic fields, identify gaps in knowledge about such effects, recommend research needs, and work towards international resolution of health concerns over the use of RF technology. The WHO maintains a Web site that provides extensive information on this project and about RF biological effects and research.

Various organizations and countries have developed exposure standards for RF energy. These standards recommend safe levels of exposure for both the general public and for workers. In the United States, the Federal Communications Commission (FCC) has adopted and used recognized safety guidelines for evaluating RF environmental exposure since 1985. Federal health and safety agencies-such as the Environmental Protection Agency (EPA), the Food and Drug Administration (FDA), the National Institute for Occupational Safety and Health (NIOSH), and the Occupational Safety and Health Administration (OSHA)-have also been involved in monitoring and investigating issues related to RF exposure.

The FCC guidelines for human exposure to RF fields were derived from the recommendations of two expert organizations, the National Council on Radiation Protection and Measurements (NCRP) and the Institute of Electrical and Electronics Engineers (IEEE). Expert scientists and engineers developed both the NCRP exposure criteria and the IEEE standard after extensive reviews of the scientific literature related to RF biological effects. The exposure guidelines are based on thresholds for known adverse effects, and they incorporate appropriate margins of safety. Many countries in Europe and elsewhere use exposure guidelines developed by the International Commission on Non-Ionizing Radiation Protection (ICNIRP). The ICNIRP safety limits are generally similar to those of the NCRP and IEEE, with a few exceptions.

The NCRP, IEEE, and ICNIRP exposure guidelines state the threshold level at which harmful biological effects may occur, and the values for maximum permissible exposure (MPE) recommended for electric and magnetic field strength and power density in both documents are based on this threshold level. The threshold level is a SAR value for the whole body of 4 watts per kilogram (4 W/kg). The most restrictive limits on whole-body exposure are in the frequency range of 30-300 MHz where the RF energy is absorbed most efficiently when the whole body is exposed. For devices that only expose part of the body, such as mobile phones, different exposure limits are specified.

Major RF transmitting facilities under the jurisdiction of the FCC-such as radio and television broadcast stations, satellite-earth stations, experimental radio stations, and certain cellular, PCS, and paging facilities-are required to undergo routine evaluation for RF compliance whenever an application is submitted to the FCC for construction or modification of a transmitting facility or renewal of a license. Failure to comply with the FCC’s RF exposure guidelines could lead to the preparation of a formal Environmental Assessment, possible Environmental Impact Statement, and eventual rejection of an application.